I've come to really enjoy webflow when working on personal projects in which I've replaced photoshop as my layout/design tool with your wonderful application. The biggest problem I have is the lack of control on the following:
Control of an element's vertical-align property
This is nice to have control to have when working with inline-block elements more often than floats.
Control of an element's cursor property
Control of the base stylings for a, h1, h2, etc.
I cannot explain how annoying it is to have to add a class or modify the header of a site to get my links looking the way I expect.
Control of the box-sizing property.
Complex layouts that are not using your build in responsive grid often make heavy use of this property, and I often find myself frustrated that I do not have access to this property on elements.
The ability to add form elements outside of webflow's form widget
If these issues were addressed I feel webflow would be better and more used as a design tool for larger, complex layouts / projects.